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Nitrogen in nitrate

The nitrate ion has charge -1, so the oxidation numbers add up to -1. Oxygen is -2, and there are three of them. Nitrogen gets the rest. A high oxidation number means that the nitrogen can be reduced, and nitrate can therefore act as an oxidising agent, especially in acidic solution. In ammonia, nitrogen is instead -3, because the hydrogens contribute +1 and the molecule is neutral. The same element can thus have very different numbers in different compounds.

N+3⋅(−2)=−1N + 3\cdot(-2) = -1the sum in the nitrate ion

Symbols

NO3−\mathrm{NO}_3^-ion charge -1
OO-2

Example

In NO3-: N+3⋅(−2)=−1N + 3 \cdot (-2) = -1, that is N−6=−1N - 6 = -1.

N=6−1=5N = 6 - 1 = 5, so the oxidation number is +5.

Include the charge of the ion in the calculation. If you forget it, nitrogen gets the wrong number.
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Part of Chemistry 1: Redox reactions.